Add missing svg source files for several icons.

The icons are:

 'PrePostCondition', 'accept_signal', 'accept_time_event',
 'combined_fragment.svg', 'exception', 'final_activity',
 'object_node', 'pin', 'precondition', 'region',
 'umbr-message-found', 'umbr-message-lost'.

Signed-off-by: Ralf Habacker <ralf.habacker@freenet.de>